Central nervous system gliomas cause morbidity and mortality by relentless growth and local invasion. High-grade gliomas typically kill patients within months after diagnosis. Despite advances in therapy, this dismal prognosis has not changed significantly. One of the hallmarks of glioma cells is their genetic instability, which contributes to progression and therapy resistance. A major barrier to the development of more effective glioma treatment strategies is the relative lack of understanding of the mechanisms underlying glioma development, progression and genetic instability. We have made a discovery that may offer a novel approach to the treatment of gliomas. Our laboratory discovered that glypican-1 (GPC1), a cell surface proteoglycan, is overexpressed in the vast majority of gliomas. Overexpression of GPC1 in vitro activates c-Myc and triggers the Skp2 autoinduction loop, which is characterized by the coordinated induction of E2F, Skp2, cyclin E and cyclin-dependent kinase 2 (CDK2) and the suppression of the cyclin-dependent kinase inhibitors (CKIs) p21 and p27. These coordinated changes in cell cycle regulators lead to G1-S-transition and DNA replication. In glioma cells and astrocytes, GPC1 overexpression also induces DNA re-replication resulting in DNA damage. Preliminary data indicate that the oncogene c-Myc plays is a major upstream mediator of these changes. We hypothesize that GPC1 is a potent regulator of the cell cycle and acts as an oncogene when overexpressed in glial cells. We propose to test this hypothesis by deciphering the signaling pathway of GPC1 in glioma cells and astrocytes and by determining the effect of GPC1 loss and overexpression on glioma development in vivo. Aim 1: Analyze the mechanism of Gpc1 in cell cycle regulation: We will analyze the effect of different GPC1 concentrations on DNA replication and cell cycle regulators in astrocytes and different glioma cells. A particular focus will be placed on GPC1 activity in the activation of c-Myc. In addition, we will employ a discovery strategy to identify GPC1 interaction partners. Aim 2: Analyze the ability of GPC1 to transform human astrocytes: Immortal and primary human astrocytes will be transduced to overexpress GPC1. The malignant potential of the cells will be assessed in in vitro transformation assay and with in vivo tumorigenesis experiments. Aim 3: Determine the role of GPC1 in glioma development and progression in vivo: The role of GPC1 in early gliomagenesis will be assessed using state-of-the-art in vivo models based on the RCAS tv-a viral receptor system. Using a gain-of-function approach, GPC1 will be overexpressed in neural progenitors and astrocytes to determine whether GPC1 is sufficient for glioma induction. Using a loss-of-function approach, genetically GPC1-deficient mice will be crossed with transgenic mice, which express activated ras oncogene in a astrocyte-specific manner and which develop high-grade gliomas with high penetrance and short latency. These experiments will reveal whether GPC1-deficient animals are protected from gliomagenesis. In addition, we will interrogate an existing tissue microarray to determine whether GPC1 overexpression is an independent prognostic marker in human glioma. Significance: The proposed studies are designed to offer novel insights into the biology of glioma development and progression and into the specific role of the proteoglycan GPC1 in these processes. The GPC1 signaling pathway may offer opportunities for therapeutic intervention by targeting either GPC1 itself or downstream mediators.